arXiv subjects

M. S. Anwar

Publications and source records attributed to M. S. Anwar.

At least 19 recordsLinked to original sources

arXiv2023

Spontaneous superconducting diode effect in non-magnetic Nb/Ru/Sr$_2$RuO$_4$ topological junctions

Non-reciprocal electronic transport in a material occurs if both time reversal and inversion symmetries are broken. The superconducting diode effect (SDE) is an exotic manifestation of this type of behavior where the critical current for positive and negative currents are mismatched, as recently observed in some non-centrosymmetric superconductors with a magnetic field. Here, we demonstrate a SDE in non-magnetic Nb/Ru/Sr$_2$RuO$_4$ Josephson junctions without applying an external magnetic field. The cooling history dependence of the SDE suggests that time-reversal symmetry is intrinsically broken by the superconducting phase of Sr$_2$RuO$_4$. Applied magnetic fields modify the SDE dynamically by randomly changing the sign of the non-reciprocity. We propose a model for such a topological junction with a conventional superconductor surrounded by a chiral superconductor with broken time reversal symmetry.

cond-mat.supr-con↗

arXiv2021

Gate-controlled conductance of superconducting NbN nanowires: coherent quantum phase-slips or Coulomb blockade?

Coherent quantum phase slips are expected to lead to a blockade of dc conduction in sufficiently narrow superconducting nanowires below a certain critical voltage. We present measurements of NbN nanowires in which not only is a critical voltage observed, but also in which this critical voltage may be tuned using a side-gate electrode. The critical voltage varies periodically as the applied gate voltage is varied. While the observations are qualitatively as expected for quantum interference between coherent quantum phase slip elements, the period of the tuning is orders of magnitude larger than expected on the basis of simple capacitance considerations. Furthermore, two significant abrupt changes in the period of the variations during measurements of one nanowire are observed, an observation which constrains detailed explanations for the behaviour. The plausibility of an explanation assuming that the behaviour arises from granular Josephson junctions in the nanowire is also considered.

cond-mat.supr-con↗

arXiv2019

Anomalous anisotropic behaviour of spin-triplet proximity effect in Au/SrRuO$_3$/Sr$_2$RuO$_4$ junctions

Spin-polarized supercurrents can be generated with magnetic inhomogeneity at a ferromagnet/spin-singlet-superconductor interface. In such systems, complex magnetic inhomogeneity makes it difficult to functionalise the spin-polarized supercurrents. However, spin-polarized supercurrents in ferromagnet/spin-triplet-superconductor junctions can be controlled by angle between magnetization and spin of Copper pairs ($d$-vector), that can effectively be utilized in developing of a field of research known as superconducting spintronics. Recently, we found induction of spin-triplet correlation into a ferromagnet SrRuO$_3$ epitaxially deposited on a spin-triplet superconductor Sr$_2$RuO$_4$, without any electronic spin-flip scattering. Here, we present systematic magnetic field dependence of the proximity effect in Au/SrRuO$_3$/ Sr$_2$RuO$_4$ junctions. It is found that induced triplet correlations exhibit strong anisotropic field response. Such behaviour attributes to the rotation of the $d$-vector of Sr$_2$RuO$_4$. This anisotropic behaviour is in contrast with the vortex dynamic. Our results will stimulate study of interaction between ferromagnetism and unconventional superconductivity.

cond-mat.supr-con↗

arXiv2019

Novel highest-Tc superconductivity in two-dimensional Nb2C MXene

Currently, superconductivity in two-dimensional (2D) materials is a hot topic of research owing to their potential technological applications. Here, we report observation of superconductivity in a 2D Nb2C MXene with transition temperature of 12.5 K, which is the highest transition temperature in MXene attained till now. We systematically optimized the chemical etching process to synthesize the Nb2C MXene from its Nb2AlC MAX phase. The X-ray diffraction (XRD) shows a clear (002) peak indicating the successful formation of MXene as well as a significant increase in the c-lattice parameter from 13.83Å to 22.72Å that indicates the delamination of Nb2C MXene sheets as revealed by morphological study using scanning electron microscope. The Meissner effect is detected using superconducting quantum interference device (SQUID: Quantum design). Lower and upper critical fields as a function of temperature follow the Ginzburg-Landau (GL) theory indicating the superconducting nature of the Nb2C MXene. Strong-electron phonon interaction and the large density-of-states at Fermi level may cause the emergence of superconductivity at such a higher transition temperature which has theoretically been predicted for Mo2C MXene. Our work is a significant advancement in the field of research and potential applications of 2D MXene.

cond-mat.supr-con↗

arXiv2019

Observation of superconducting gap spectra of long-range proximity effect in Au/SrTiO$_3$/SrRuO$_3$/Sr$_2$RuO$_4$ tunnel junctions

We observe an unconventional superconducting minigap induced into a ferromagnet SrRuO$_3$ from a spin-triplet superconductor Sr$_2$RuO$_4$ using a Au/SrTiO$_3$/SrRuO$_3$/Sr$_2$RuO$_4$ tunnel junction. Voltage bias differential conductance of the tunnel junctions exhibits V-shaped gap features around zero bias, corresponding to a decrease in the density-of-states with an opening of a superconducting minigap in SrRuO$_3$. Observation of a minigap at a surface of a 15~nm thick SrRuO$_3$ layers confirms the spin-triplet nature of induced superconductivity. The shape and temperature dependence of the gap features in the differential conductance indicate that the even-frequency $p$-wave correlations dominate, over odd-frequency $s$-wave correlations. Theoretical calculations support this $p$-wave scenario. Our work provides the density-of-states proof for $p$-wave Cooper pair penetration in a ferromagnet and significantly put forward our understanding of the $p$-wave spin-triplet proximity effect between spin-triplet superconductors and ferromagnets.

cond-mat.supr-con↗

arXiv2018

Emergence of Quantum Phase-Slip Behaviour in Superconducting NbN Nanowires: DC Electrical Transport and Fabrication Technologies

Superconducting nanowires undergoing quantum phase-slips have potential for impact in electronic devices, with a high-accuracy quantum current standard among a possible toolbox of novel components. A key element of developing such technologies is to understand the requirements for, and control the production of, superconducting nanowires that undergo coherent quantum phase-slips. We present three fabrication technologies, based on using electron-beam lithography or neon focussed ion-beam lithography, for defining narrow superconducting nanowires, and have used these to create nanowires in niobium nitride with widths in the range of 20-250 nm. We present characterisation of the nanowires using DC electrical transport at temperatures down to 300 mK. We demonstrate that a range of different behaviours may be obtained in different nanowires, including bulk-like superconducting properties with critical-current features, the observation of phase-slip centres and the observation of zero conductance below a critical voltage, characteristic of coherent quantum phase-slips. We observe critical voltages up to 5 mV, an order of magnitude larger than other reports to date. The different prominence of quantum phase-slip effects in the various nanowires may be understood as arising from the differing importance of quantum fluctuations. Control of the nanowire properties will pave the way for routine fabrication of coherent quantum phase-slip nanowire devices for technology applications.

cond-mat.supr-con↗

arXiv2017

Anisotropic magnetothermoelectric power of ferromagnetic thin films

We compare the behavior of the magnetothermoelectric power (MTEP)in metallic ferromagnetic thin films of Ni80Fe20 (Permalloy; Py), Co and CrO2 at temperatures in the range of 100 K to 400 K. In 25 nm thick Py films and 50 nm thick Co films both the anisotropic magnetoresistance (AMR) and MTEP show a relative change in resistance and thermoelectric power (TEP) of the order of 0.2% when the magnetic field is reversed, and in both cases there is no significant change in AMR or MTEP any more after the saturation field has been reached. Surprisingly, both Py and Co films have opposite MTEP behavior although both have the same sign for AMR and TEP. The data on 100 nm films of fully spin-polarized CrO2, grown both on TiO2 and on sapphire, show a different picture. The MTEP behavior at low fields shows peaks similar to the AMR in these films, with variations up to 1%. With increasing field both the MR and the MTEP variations keeps growing, with MTEP showing relative changes of 1.5% with the thermal gradient along the b-axis and even 20% with the gradient along the c-axis, with an intermediate value of 3% for the film on sapphire. It appears that the low-field effects are due to magnetic domain switching, while the high-field effects are intrinsic to the electronic structure of CrO2.

cond-mat.mtrl-sci↗

arXiv2017

Multi-component order parameter superconductivity of Sr$_2$RuO$_4$ revealed by topological junctions

Single crystals of the Sr$_2$RuO$_4$-Ru eutectic system are known to exhibit enhanced superconductivity at 3~K, in addition to the bulk superconductivity of Sr$_2$RuO$_4$ at 1.5~K. The 1.5-K phase is believed to be a spin-triplet, chiral $p$-wave state with the multi-component order parameter, giving rise to chiral domain structure. In contrast, the 3-K phase is attributable to enhanced superconductivity of Sr$_2$RuO$_4$ in the strained interface region between Ru inclusion of a few to tens of micrometers in size and the surrounding Sr$_2$RuO$_4$. We investigate the dynamic behavior of a topological junction, where a superconductor is surrounded by another superconductor. Specifically, we fabricated Nb/Ru/Sr$_2$RuO$_4$ topological superconducting junctions, in which the difference in phase winding between the $s$-wave superconductivity in Ru micro-islands induced from Nb and the superconductivity of Sr$_2$RuO$_4$ mainly governs the junction behavior. Comparative results of the asymmetry, hysteresis and noise in junctions with different sizes, shapes, and configurations of Ru inclusions are explained by the chiral domain-wall motion in these topological junctions. Furthermore, a striking difference between the 1.5-K and 3-K phases is clearly revealed: the large noise in the 1.5-K phase sharply disappears in the 3-K phase. These results confirm the multi-component order-parameter superconductivity of the bulk Sr$_2$RuO$_4$, consistent with the chiral $p$-wave state, and the proposed non-chiral single-component superconductivity of the 3-K phase.

cond-mat.supr-con↗

arXiv2016

Direct penetration of spin-triplet superconductivity into a ferromagnet in Au/SrRuO3/Sr2RuO4 junctions

Efforts have been ongoing to establish superconducting spintronics utilizing ferromagnet/superconductor heterostructures1. Previously reported devices are based on spin-singlet superconductors (SSCs), where the spin degree of freedom is lost. Spin-polarized supercurrent induction in ferromagnetic metals (FMs) is achieved even with SSCs, but only with the aid of interfacial complex magnetic structures, which severely affect information imprinted to the electron spin. Use of spin-triplet superconductors (TSCs) with active spins potentially overcomes this difficulty and further leads to novel functionalities. Here, we report spin-triplet superconductivity induction into a FM SrRuO3 from a leading TSC candidate Sr2RuO4, by fabricating microscopic devices using an epitaxial SrRuO3/Sr2RuO4 hybrid. The differential conductance, exhibiting Andreev-reflection features with multiple energy scales up to around half tesla, indicates the penetration of superconductivity over a considerable distance of 15 nm across the SrRuO3 layer without help of interfacial complex magnetism. This demonstrates the first FM/TSC device exhibiting the spin-triplet proximity effect.

cond-mat.supr-con↗

arXiv2015

Ferromagnetic SrRuO3 thin-film deposition on a spin-triplet superconductor Sr2RuO4 with highly conducting interface

Ferromagnetic SrRuO3 thin films are deposited on the ab-surface of single crystals of the spin-triplet superconductor Sr2RuO4 as substrates using pulsed laser deposition. The films are under a severe in-plane compressive strain. Nevertheless, the films exhibit ferromagnetic order with the easy axis along the c-direction below the Curie temperature of 158 K. The electrical transport reveals that the SrRuO3/Sr2RuO4 interface is highly conducting, in contrast with the interface between other normal-metals and the ab-surface of Sr2RuO4. Our results will stimulate the investigations on proximity effects between a ferromagnet and a spin-triplet superconductor.

cond-mat.supr-con↗

arXiv2013

Anomalous switching in Nb/Ru/Sr2RuO4 topological junctions by chiral domain wall motion

A spontaneous symmetry breaking in a system often results in domain wall formation. The motion of such domain walls is utilized to realize novel devices like racetrack-memories, in which moving ferromagnetic domain walls store and carry information. Superconductors breaking time reversal symmetry can also form domains with degenerate chirality of their superconducting order parameter. Sr2RuO4 is the leading candidate of a chiral p-wave superconductor, expected to be accompanied by chiral domain structure. Here, we present that Nb/Ru/Sr2RuO4 topological superconducting-junctions, with which the phase winding of order parameter can be effectively probed by making use of real-space topology, exhibit unusual switching between higher and lower critical current states. This switching is well explained by chiral-domain-wall dynamics. The switching can be partly controlled by external parameters such as temperature, magnetic field and current. These results open up a possibility to utilize the superconducting chiral domain wall motion for future novel superconducting devices.

cond-mat.supr-con↗

arXiv2013

Anomalous transport in half-metallic ferromagnetic CrO2

We have investigated transport properties of CrO2 thin films deposited on TiO2 and sapphire substrates. The films are good metals down to low temperatures. The residual resistivity is of the order of 6 micro-ohhmcm for films deposited on TiO2 and two times higher for films on sapphire substrates. The sign of the magnetoresistance (MR) changes from negative to positive at a temperature around 100 K. This fact, as well as a rapid change in the ordinary and anomalous Hall coefficients suggest a change in the electronic state. At lower temperatures the MR is a linear function of the applied field. This linear dependence might be explained as intergrain tunneling MR. This interpretation is also suggested by the angular MR. The planar Hall effect measurements reveal that the CrO2 thin films are not in a single magnetic domain state even for films deposited on an isostructural TiO2 substrate.

cond-mat.str-el↗

arXiv2011

Long range spin supercurrents in ferromagnetic CrO$_2$ using a multilayer contact structure

e report measurements of long ranged supercurrents through ferromagnetic and fully spin-polarized CrO$_2$ deposited on TiO$_2$ substrates. In earlier work, we found supercurrents in films grown on sapphire but not on TiO$_2$. Here we employed a special contact arrangement, consisting of a Ni/Cu sandwich between the film and the superconducting amorphous Mo$_{70}$Ge$_{30}$ electrodes. The distance between the contacts was almost a micrometer, and we find the critical current density to be significantly higher than found in the films deposited on sapphire. We argue this is due to spin mixing in the Ni/Cu/CrO$_2$ layer structure, which is helpful in the generation of the odd-frequency spin triplet correlations needed to carry the supercurrent.

cond-mat.supr-con↗

arXiv2010

Supercurrents through half-metallic ferromagnetic CrO$_2$ revisited

We report on the observation of a supercurrent through the half metallic ferromagnet CrO$_2$ grown on hexagonal Al$_2$O$_3$ (sapphire). The current was observed to flow over a distance of 700 nm between two superconducting amorphous Mo$_{70}$Ge$_{30}$ electrodes which were deposited on the CrO$_2$ film. The critical current $I_c$ increases as function of decreasing temperature. Upon applying an in-plane magnetic field, $I_c$ goes through a maximum at the rather high field of 80 mT. We believe this to be a long range proximity effect in the ferromagnet, carried by odd-frequency pairing correlations.

cond-mat.supr-con↗

arXiv2005

Sharing Polarization within Quantum Subspaces

Given an ensemble of n spins, at least some of which are partially polarized, we investigate the sharing of this polarization within a subspace of k spins. We assume that the sharing results in a pseudopure state, characterized by a single purity parameter which we call the bias. As a concrete example we consider ensembles of spin-1/2 nuclei in liquid-state nuclear magnetic resonance (NMR) systems. The shared bias levels are compared with some current entanglement bounds to determine whether the reduced subspaces can give rise to entangled states.

quant-ph↗

arXiv2005

NMR Quantum Information Processing with Para-Hydrogen

This thesis addresses the problems of initialization and separability in liquid state NMR based quantum information processors. We prepare pure quantum states lying above the entanglement threshold. Our pure state quantum computer derives its purity from the highly polarized nuclear spin states in the para-hydrogen molecule. The thesis begins with a critique of conventional NMR based quantum information processing outlining the major strengths and weaknesses of the technology. We describe the enhanced magnetic ordering of the nuclear spin states in para-hydrogen and an initialization experiment exploiting this effect to achieve pure, entangled states. These states can indeed be used as initial states in implementing quantum algorithms: we describe mplementations of the Deutsch and the Grover quantum algorithms. The "twirl" operation converts a completely arbitrary input state to a Werner singlet. The NMR implementation of this operation is taken up. We also analyze the possibility of sharing the purity of some highly polarized qubits in a quantum computer onto quantum subspaces of arbitrary dimensions, and whether these sharing operations increase or decrease the likelihood of entanglement.

quant-ph↗

arXiv2004

Practical Implementations of Twirl Operations

Twirl operations, which convert impure singlet states into Werner states, play an important role in many schemes for entanglement purification. In this paper we describe strategies for implementing twirl operations, with an emphasis on methods suitable for ensemble quantum information processors such as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl operation on a general two-spin mixed state using liquid state NMR techniques, demonstrating that we can obtain the singlet Werner state with high fidelity.

quant-ph↗

arXiv2004

Implementing Grover's Quantum Search on a Para-Hydrogen based Pure State NMR Quantum Computer

We demonstrate the implementation of Grover's quantum search algorithm on a liquid state nuclear magnetic resonance (NMR) quantum computer using essentially pure states. This was achieved using a two qubit device where the initial state is an essentially pure ($ε=1.06\pm0.04$) singlet nuclear spin state of a pair of 1H nuclei arising from a chemical reaction involving para-hydrogen. We have implemented Grover's search to find one of four inputs which satisfies a function.

quant-ph↗